(Note: We have used s at this point for the semiperimeter of the TRIANGLE.
In what follows, we will substitute for s, e, and f in terms of a, b, c,
and d. Eventually we will return to the use of s to represent the semiperimeter
of the quadrilateral.)

1. First, we want a substitution for e in terms of a, b, c, and d.

2. Next, we want a substitution for f in terms of a, b, c, and d.

3. Now we will make some substitutions in the formulas for the triangle.

4. Now evaluate s(s - (e + a)

5. Likewise, we now evaluate s - (f + c) in terms of a, b, c, and d.

6. Now, we evaluate (s - b)

7. Now we are ready to evaluate the Area of the quadrilateral in terms
of a, b, c, d.